Corrugated shipping containers are used to package, store and ship a myriad of products, e.g. from potato chips on the low density end to ball bearings on the high density end of the range. Filled corrugated shipping containers are stacked for storage or transport to as high a height as practical to make optimum use of truck and warehouse space. Typically, lower density materials are packed in large containers made of light wall corrugated board and, conversely, higher density materials are packed in small containers made of stronger wall corrugated board. The ultimate limitation of corrugated wall strength is how many additional filled cartons can be stacked on top of a bottom carton before the bottom carton collapses.
Corrugated shipping containers are made of corrugated board. Corrugated board is produced by feeding three sheets of paper into a machine in parallel layers, with the middle layer fed at a greater speed than the two outer layers. The middle layer is alternately bent upward and downward to become a sinusoidal wave, or rippled form, also known as flutes. The upper and lower layers are kept flat and adhered to the peaks of the middle layer. For greater load bearing strength, double wall corrugated boards are used to make containers. A double wall corrugated board has three flat sheets interspersed with two rippled sheets, creating a heavy and strong composite. The rippled sheets may be equal in peak height or different.
Most corrugated containers are three dimensional square or rectangular boxes. Conventional container construction, as well as limitation of corrugated board manufacturing equipment, dictate that the corrugations in the traditional finished box are oriented vertically. Also, vertical corrugations serve as substantially rigid columns, increasing the weight bearing capacity of the board. In the plastics industry, large cartons variously known as bulk bins or gaylords are used for shipping and storing granulated plastic resin. The plastic resin granules are later melted and formed by molding or extruding into plastic products. These bulk bins are generally made of double wall corrugated board and may be loaded with up to 1800 pounds of resin pellets. In contrast to smaller corrugated containers where the box top is closed by folding four integral flaps, bulk bins are usually closed by a separate tray-like lid that is placed on the filled container bottom. Because the resin granules are small and smooth, a volume of granules tends to act as a quasi-liquid, i.e. the weight forces lower portions of the pellet mass to expand laterally against the bulk bin wall, causing the bulk bin to bulge outward. When bulging occurs, the columnar weight bearing strength of the wall is diminished, increasing the bulging further.
In order to reduce the tendency of the walls to bulge, many bulk bins have been made in an octagonal cross sectional shape, as viewed from above, to reduce the lateral wall length and increase the effective stiffness of the wall. While this octagonal shape reduces the bulging and makes the shipping containers more reliable, the octagonal shape detracts from the weight of plastic pellets that a bulk bin can carry and increases the ultimate storage space required for each ton of pellets.